The present invention relates generally to the dental alloys which are used for filling teeth from which decayed portions have been removed. More particularly, the invention relates to a corrosion resistant dental alloy which permits adjusting the handling characteristics of amalgams made with such alloy.
The prior art emphasized the development of alloys which are corrosion resistant. While typical dental alloys are principally composed of silver and tin, they usually contain small amounts of copper and zinc. A typical alloy of the prior art would contain at least 65 wgt.% silver, about 1-2 wgt.% zinc, and about 2-4 wgt.% copper, with the remainder being tin. Such alloys are not completely resistant to corrosion. It has been found that increasing the copper content of such alloys provides increased strength and also avoids the formation of what is known in the art as the gamma-two phase, a tin and mercury phase which has low resistance to corrosion and thus may lead to early deterioration of fillings. Typical of such high copper alloys are those disclosed in U.S. Pat. No. 3,871,876 and U.S. Pat. No. 3,997,328. Such dental alloy compositions increase the copper content from the typical 2-4 wgt.% to the range of 8-27 wgt.% in the first mentioned patent, and in the latter patent, from 20-40 wgt.%.
While such alloys have improved corrosion resistance, another important characteristic of dental alloys has been neglected heretofore. The success of a dentist in filling a dental cavity is related to the handling characteristics of the alloy after it is amalgamated with mercury. For example, the high copper alloy disclosed in U.S. Pat. No. 3,871,876, is typically produced by air atomization from the molten state which results in a spherical or spheriodal form for the finished alloy. It is characteristic of alloys having a spherical shape that they feel relatively soft to the dentist and appear to require delicate handling. They are sometimes difficult to pack into a dental cavity since they have a tendency to be formed up the wall of the cavity if too much pressure is exerted or an instrument is used which has a small bearing area. Consequently, many dentists find that such spherical material is not well-adapted to their individual technique. As a result, they may be unable to take advantage of the corrosion resistance inherent with spherical alloys having a high copper content.
One method of improving handling characteristics of conventional dental alloys is disclosed and claimed in U.S. Pat. No. 3,997,327. In that invention a major portion of spherical particles is combined with a minor portion of microcut irregular particles, or flakes. Typical dental alloys in the prior art generally have been of the flake type, which inherently requires a higher pressure in order to pack it into a dental cavity than is characteristic of the spherical particles. By combining spherical particles with flake particles having the same or a similar composition, it is possible to improve the handling characteristics of the resulting mixture. Such a combination, having a conventionally low copper content, has less resistance to corrosion than the higher copper content alloys previously discussed.
Another method of improving handling characteristics applied to a corrosion resistant alloy is disclosed and claimed in a copending application filed concurrently with the present application in the name of the inventors hereof and assigned to a common assignee, entitled "Corrosion-Resistant Dental Alloy Having Improved Handling Characteristics" (hereinafter "Improved Alloy"). The alloy of U.S. Pat. No. 3,871,876 is produced as particles having a unique randomly-shaped microcrystalline morphology which is characterized by having a higher than average silver and copper content at the surface of the particles than in the interior of the particles and a surface area about 20-30% greater than that of spherical particles and about 20-30% less than that of flake-like particles. The unique form of the particles provides handling characteristics similar to those of flake-like particles, while retaining the corrosion resistance characteristic of the spherical form. Although the unique particles of Improved Alloy are advantageous in providing excellent handling characteristics, fine adjustment of the handling characteristics is not readily made with it.
The present invention has as its objective providing adjustment of handling characteristics of corrosion-resistant dental alloys.